Gas/liquid separation in a hydrocarbon conversion process

ABSTRACT

A gas/liquid(s) separation system is constituted by three different sections: a primary separator ( 1 ) for flows with a G/L in the range 0.1 to 10; a secondary separator ( 2 ) for flows with a G/L in the range 10 to 50; a system ( 3 ) which limits the formation of a liquid vortex; where G/L is the ratio of the gas to liquid mass flow rates. The separation system is suitable for the separation of gas and liquid from a hydrocarbon conversion process, e.g. a hydrotreatment process.

The present invention is applicable to refining or petrochemicalprocesses and in general to any conversion simultaneously using a liquidphase—or at least one hydrocarbon—and a gas phase—a mixture of hydrogenand hydrocarbon vapour fractions—in thermodynamic equilibrium with theliquid phase. The invention relates to the field of processesfunctioning, for example, with a ratio of gas to liquid mass flow rates,G/L, normally in the range 0.1 to 10, usually in the range 0.5 to 2. Itis of particular application to hydrotreatment processes.

The particular aim of the process is to convert at least a portion of ahydrocarbon feed, for example an atmospheric residue obtained bystraight run distillation of a crude petroleum, into light gasoline andgas oil fractions and into a heavier product which can be used as a feedfor a more selective conversion process such as fluidised bed catalyticcracking, for example after an intermediate deasphalting step(extraction of asphaltenes using a C3-C7 solvent). The process may alsobe aimed at converting a distillate obtained by vacuum distillation ofan atmospheric residue from crude petroleum into light gasoline and gasoil fractions and into a heavier product which can be used as a feed ina more selective conversion process such as fluidised bed catalyticcracking. The invention also has application in processes forhydrotreating heavy or light hydrocarbon feeds, such ashydrodesulphurisation, hydrodenitrogenation or hydrodearomatisationprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic representations of different arrangements ofthe separation system of the invention; and

FIG. 3 shows a separation system with three different zones or sections.

There are two types of arrangements for the proposed separator (FIG. 1).Separator (200), which is placed downstream of the liquid/gas outlet(300) from the reactor (100), can either be placed in the liquidrecycling circuit of the reactor, or it can be placed in the finaloutlet from the reactor. The liquid from separation can then either bereturned to the reactor via the recycle circuit (500), the product thenbeing withdrawn downstream of (200) at (700), or it can constitute theproduct extraction from the process. The separated gas is evacuated via(400).

The residence time in the liquid separation system (200) is in the range30 seconds to 10 minutes, usually in the range 1 to 3 minutes, forexample close to 2 minutes. The range of the ratio of the gas to liquidmass flow rates, G/L, is in the range 0.1 to 10, preferably in the range0.5 to 2. The liquid mass flow rates in the inlet line (300) at theseparator inlet (200) are generally in the range 100 to 4000 kg/s/M².These gas mass flow rates are usually in the range 100 to 800 kg/s/m² inthe separator inlet line (300).

The fluid temperature is generally in the range 20° C. to 600° C.,preferably in the range 300° C. to 450° C., and the operating pressurecan be in the range 1 to 200 bars. The dynamic viscosity of the gas isin the range 10⁻² to 2×10⁻² cP, that of the liquid is in the range 0.3to 5 cP. The surface tension is in the range 20 to 70 mN/m. The liquiddensity is generally in the range 500 to 1000 kg/m³, usually in therange 500 to 700 kg/m³. The gas density is normally in the range 1 to 50kg/m³, usually in the range 30 to 50 kg/m³.

One of the original features of this separation process is that it canfunction properly at low values of Δρ=ρ_(L)−ρ_(G) (close to 500 kg/m³)and over a wide range of mass flow ratios G/L (in the range 0.1 to 10).

The system can continue to function when the liquid phase contains solidparticles of an organic or mineral nature.

DESCRIPTION OF THE SYSTEM AND ASSOCIATED PROCESS

The process associated with the present invention is intended, forexample, to treat a vacuum distillate from a zone for vacuumdistillation of a crude petroleum. The hydrotreatment process (FIG. 2)generally functions in the presence of hydrogen and comprises at leastone three-phase reactor (100) containing a hydrotreatment catalyst whichconverts in an ebullated bed (20), generally functioning in liquid andgas upflow mode. The reactor preferably comprises at least one means(50) for extracting catalyst from said reactor located close to the baseof the reactor and at least one means (40) for adding fresh catalystclose to the top of said reactor. Said reactor comprises at least onecircuit for recycling liquid phase (60), located inside or outside thereactor, and intended to maintain a sufficient degree of expansionnecessary for the bed to function in a three-phase ebullated operation.At the top of the reactor, downstream of the bed expansion, an axialgas/liquid separation system inside the reactor can separate the liquidphase to be recycled (70). The level of the liquid in this internalseparator is maintained by line (300), allowing the gas phase to escapeand withdrawing liquid phase products. The flow of these two phases thenenters the separator of the present invention (200).

Excellent gas-liquid separation must be achieved downstream of thereactor. If liquid is entrained at the gas outlet, this can generateprocess fluctuations in the heat exchangers. Similarly, if a gasfraction is withdrawn from the liquid outlet in the form of pockets,this will cause pressure peaks in the lines downstream of the separatorwhich will destabilise the steady flow of the products. Thesedysfunctions are thus deleterious to the operation of the units locateddownstream of the (gas/liquid separator. In the case of hydrotreatmentor hydroconversion units, this also leads to an expensive loss ofhydrogen, and it is thus vital to provide a system which can permitrapid and effective separation of the two liquid and gas phasesassociated with good regulation of the liquid level in the vessel.

In order to limit exposure of the liquid to the high temperature outsidethe reactor to limit thermal degradation, the residence time for theliquids must be limited.

Thus the invention proposes an effective gas-liquid separation processwhich can enable liquid to be evacuated rapidly, characterized in thatthis system remains effective when the liquid-gas density difference issmall (400-1000 kg/m³).

This process for separating liquid(s) and gas originating from ahydrocarbon conversion zone is carried out in a zone generallycomprising three successive sections; the first section, the primarysection, functioning for flows with a G/L in the range about 0.1 to 10;the second section, the secondary section, functioning for flows with aG/L in the range about 10 to 50, and the third section acting to limitthe formation of a liquid vortex, where G/L is the ratio of the gas toliquid mass flow rates.

The present invention also concerns a separation system or apparatusconstituted by three different sections (see FIG. 3):

a primary separator (1) for flows with a G/L in the range 0.1 to 10,

a secondary separator (2) for flows with a G/L in the range 10 to 50;

a system which limits the formation of a liquid vortex (3).

The dimensions of the vessel (200) and the position imposed on thenormal level of liquid in vessel (200) are determined so as to impose aresidence time in the range 1 to 10 minutes, preferably close to 2minutes.

The separator must achieve a separation efficiency such that no morethan 0.1% to 0.5% maximum (by weight) of liquid remains in the gas phaseat the separator outlet and no more than 0.5% to 1% maximum (by weight)of gas remains in the liquid phase at the separator outlet.

The primary separator (1) is preferably constituted by a tube terminatedby at least one tangential outlet, causing the flow to rotate through90° at the tube outlet. As an example, the ratio of the area of theopenings in each tangential outlet and the area of the cross section offlow in the tube is in the range 0.25 to 1, preferably 0.5. The ratiobetween the height and width of each opening is in the range 1 to 4,preferably 2. Inside the tube, upstream of the tangential outlets, ahelix can be added. This helix (5) can be a single or double helix. Theratio of the helix width, corresponding to the cross section of flow ofthe fluids, to the tube diameter is generally in the range 0.5 to 1. Thepitch number of the helix (i.e., the ratio of the total height to thehelix pitch) is generally in the range 1 to 6, and preferably in therange 2 to 3.

The primary separator (1) is traversed by the whole of the gas/liquidflow entering the separator. The efficacy of this separator is generallyin the range 70% to 90% on the gas outlet side. The gas flow producedfrom the gas/liquid separation in the primary separator (1) is directedtowards the secondary separator (2).

The secondary separator (2) is constituted by a cyclone with a freetangential inlet (6). As an example, the tangential inlet (6) has arectangular cross section, and the ratio of the width to the height ofthis cross section is in the range 0.2 to 0.6, preferably close to 0.5.The ratio of the cross sectional area of the inlet to the crosssectional area of the cyclone (2) is generally in the range 0.06 to0.25, usually close to 0.12. The ratio of the diameter of the gas flowoutlet line (8) to the cyclone diameter is generally in the range 0.3 to0.6, preferably close to 0.5. This ratio must be maximised so as toreduce the ΔP (difference in pressure between the inlet and outlet ofthe cyclone) in the cyclone. The ratio of the height of the gas outletline (8) to the cyclone diameter is normally in the range 0 to 1,usually close to 0.5. The liquid outlet from this cyclone (7 a) isalways below the liquid level in the separator vessel (200). The liquidoutlet from the cyclone (7) is the same diameter as the cyclone andcomprises blades attached to the walls. These blades are distributed ata constant angular spacing and there are 2 to 8 or them, for example 4.The ratio of the width of these blades to the cyclone diameter is in therange 0.15 to 1, preferably close to 0.3. The height of the blades isdefined so as to leave a distance between the top of the cyclone and thetop of the blades in the range 2 to 5 diameters of the cyclone,preferably 2 to 3 diameters of the cyclone, and the blades extend to thebottom of the cyclone. The depth of the liquid in vessel (200) mustcorrespond to a minimum of the base of the outlet cone of vessel (200)and to the base of the cyclone. The maximum height of the liquid levelmust be less than 3 diameters of the cyclone from the top of thecyclone, preferably less than 4 to 6 diameters from the top of thecyclone so as to accommodate the pressure drop in the cyclone. Thevertical distance separating the outlet (4) from the primary separatorand the inlet (6) to the secondary separator must be greater than twicethe height of the rectangular inlet (6), the tangential inlet to thecyclone separator being located above the tangential outlet (4) from theprimary separator.

The combination of two separators (1) and (2) enables a good compactnessto be obtained for the whole of the separator and in particular canlimit the diameter of the vessel (200). Since the height of the cycloneacts only on the residence time of the gas, the proposed gas/liquidseparator system can thus keep the residence time for the liquid low.

The base of the vessel (200) of the separator comprises a system (3) forpreventing any formation of a vortex in the liquid phase to limit anygas from becoming entrained in the liquid outlet. This system iscomposed of blades (9) attached to the walls distributed at a constantangular spacing to dissipate the angular movement. There are 2 to 8 ofthese blades, preferably 4. The height of these blades is in the rangebetween the maximum depth of the liquid and the bottom portion of theprimary separator (1). The ratio between the width of these blades andthe diameter of the vessel is in the range 0.02 to 0.1, preferably closeto 0.05. In order to reduce the length and energy of the core of thevortex, a cylinder can be added at the base of the vessel (200), in theaxis of flow of the outlet liquid. This cylinder will have the samediameter as the liquid outlet line and a height in the range 0.5 to 2diameters of the liquid outlet line. This cylinder can have massivewalls or a wall constituted by a screen and in that case it may beclosed at its upper portion.

What is claimed is:
 1. A process for separating a mixture of liquid(s)and gas, said mixture originating from a hydrocarbon conversion zone,said process comprising: performing separation in a separation zonecomprising three successive sections; wherein the first section, theprimary section, functions for flows with a ratio of gas to liquid massflow rates, G/L, in the range of 0.1 to 10; the second section, thesecondary section, functions for flows with a ratio of gas to liquidmass flow rates, G/L, in the range of 10 to 50; and the third sectionacts to limit the formation of a liquid vortex.
 2. A process accordingto claim 1, in which the residence time in the separation zone (200) isin the range of 30 seconds to 10 minutes, the range of the ratio of thegas to liquid mass flow rates, G/L, is in the range of 0.1 to 10, at theinlet, and the liquid mass flow rates in the separator inlet line is inthe range of 100 to 4000 kg/s/m².
 3. A process according to claim 2,wherein the fluid temperature is in the range of 20° C. to 600° C.; theoperating pressure is in the range of 10⁻² to 2×10⁻² cP; the dynamicviscosity of the liquid is in the range of 0.3 to 5 cP; the surfacetension is in the range of 20 to 70 mN/M; the density of the liquid isin the range of 500 to 1000 kg/m³, and the density of the gas is in therange of 1 to 50 kg/m³.
 4. A process according to claim 1, wherein saidmixture of liquid and gas is an effluent originating from ahydrotreatment process, functioning in the presence of hydrogen, saidhydrotreatment process comprising at least one three-phase reactorcontaining a reaction zone containing hydrotreatment catalyst in anebullated bed functioning in liquid and gas upflow mode, the reactionzone comprising at least one means for extracting catalyst from saidreactor located close to the bottom of the reactor and at least onemeans for adding fresh catalyst close to the top of said reactor, saidreaction zone further comprising at least one liquid phase recyclingcircuit, located inside or outside the reactor, for maintaining asufficient degree of expansion of the bed to function in a three-phaseebullated operation, the process further comprising, at the top of thereactor and downstream of the bed expansion, an axial gas/liquidseparation system inside the reactor for separating the liquid phase tobe recycled, the level of liquid in said axial gas/liquid separatorsystem being maintained by a line intended for the escape of gas phase,and for withdrawing liquid phase products, the flow of these two phasesthen entering said separation zone.
 5. A process according to claim 1,wherein the hydrocarbon conversion zone is a hydrotreatment zone.
 6. Aprocess according to claim 1, wherein the efficiency of said process issuch that no more than 0.1%-0.5% by weight of liquid remains in the gasphase at the separator outlet and no more than 0.5%-1% by weight of gasremains in the liquid phase at the separator outlet.
 7. A processaccording to claim 1, wherein the residence time in the liquidseparation system is 1-3 minutes.
 8. A process according to claim 1,wherein the liquid mass flow rates at the separator inlet are in therange of 100-4,000 kg/s/m² and the gas flow mass flow rates at theseparator inlet are in the range of 100-800 kg/s/m².
 9. A processaccording to claim 3, wherein the density of the liquid is 500-700 kg/m³and the density of the gas is 30-50 kg/m³.
 10. A process of claim 1,wherein the density differente between the liquid and gas is 400-1,000kg/m³.
 11. A process according to claim 1, wherein in the primarysection said flow rate G/L is in the range of 0.5 to
 2. 12. A processaccording to claim 3, wherein said temperature is in the range of 300°C. to 450° C.
 13. A process for separating a mixture of liquid(s) andgas originating from a hydrocarbon conversion zone in a verticalcylindrical vessel comprising an inlet nozzle, a liquid outlet at abottom part of the vertical cylindrical vessel and a vertically disposedcyclone having an inlet and bottom and top outlets, said processcomprising a first zone wherein the mixture has a ratio of gas to liquidmass flow rate (G/L) in the range of about 0.1 to 10, passing themixture into the vessel and out of the inlet nozzle so as to createwithin the vessel a tangential flow, thereby separating a first liquidflow from the mixture to form a pool of liquid in the bottom portion ofthe vessel; in a second zone, passing residual mixtures having a G/L inthe range of about 10 to 50 into said cyclone so as to remove liquiddepleted gas from the vessel and to separate further liquid from saidbottom cyclone outlet at a rate of flow combined with said first liquidflow to provide a liquid level such that the bottom cyclone outlet isimmersed in the pool of liquid; and a third zone comprising passingliquid at the bottom part of the vessel over obstructions so as toimpede tangential flow and limit the formation of a liquid vortex.
 14. Aprocess according to claim 13, wherein said third zone comprises passingliquid over plurality of blades extending from the interior surface ofthe vessel into the pool of liquid.
 15. A process according to claim 13,wherein the hydrocarbon conversion zone is a hydrotreatment zone.